1. Field of the Invention
The present invention relates to an exhaust device for a diesel vehicle.
2. Description of Related Art
An exhaust device for a diesel vehicle is equipped with a variety of diesel after-treatment devices including a Diesel Oxidation Catalyst (DOC), a Diesel Particulate Filter (DPF), a Selective Catalytic Reduction (SCR) catalyst, a Lean NOx Trap (LNT), etc. The diesel after-treatment devices also include a hydrocarbon SCR(HC-SCR) catalyst using diesel fuel as a reducing agent.
The DOC mainly serves to oxidize hydrocarbons and CO, and is used to oxidize soot in a Diesel Particulate Filter (DPF) by oxidizing NO into NO2 or to obtain a specific ratio of NO to NO2, which can raise the efficiency of urea SCR.
In addition, the urea SCR, the most common of SCR methods, generally uses Fe or the like. The efficiency of the urea SCR is sensitive to the NO/NO2 ratio and is highest when the NO/NO2 ratio is on the order of 1.
Also, the LNT is advantageous in that a system for supplying a reducing agent is not necessary since fuel is supplied as a reducing agent even though its efficiency is lower than that of the urea SCR.
Accordingly, the urea SCR and the LNT are widely used in after-treatment devices for diesel automobiles due to high purification performance.
FIG. 1 is a schematic diagram illustrating a conventional exhaust device for a diesel vehicle.
The conventional exhaust device for a diesel vehicle includes a DOC 20 connected to an engine 10 and a HC-SCR catalyst 30 connected to the DOC 20.
The DOC 20 mainly serves to oxidize hydrocarbons and CO, and is used to obtain a ratio of NO to NO2 set to 1:1, which can raise the efficiency of a HC-SCR catalyst 30.
The HC-SCR catalyst 30 reduces NOx by controlling the ratio of hydrocarbon to NOx to be 5:1 or more by secondary fuel injection or engine post-injection in order to activate a reaction.
However, the afore-mentioned conventional HC-SCR catalyst 30 degrades its performance since it cannot maintain the ratio of hydrocarbon to NOx at 5:1 in the event where a diesel vehicle rapidly accelerates, decelerates, or changes gears, or descends down right after having climbed a hill.
Furthermore, regarding that the activation temperature of the HC-SCR catalyst 30 is typically 300° C. or more, the exhaust gas from the diesel automobile is relatively cool. This, as a result, causes another problem of poor purification performance.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.